Vacuum type circuit interrupter



y 6, 1970 J. w. RANHEIM 3,514,559

VACUUM TYPE CIRCUIT INTERRUPTER Filed March 27. 1967 2 Sheets-Sheet 1 INV EN TOR.

{5/727 2% Ran/2494772 EM Wm r/fttorney 5, 970 J. w. RANHEIM 3,514,559

VACUUM TYPE CIRCUIT INTERRUPTEF.

Filed March 27, 1967 2 Sheets-Sheet 2 m ZPB n5 1 r ,7 H V W a f /)k\ 30INVENTOR. ag/m 2M Ran/765272 United States Patent 3,514,559 VACUUM TYPECIRCUIT INTERRUPTER John W. Ranheim, Oak Creek, Wis., assignor toMcGraw- Edison Company, Milwaukee, Wis., a corporation of DelawareFiledMar. 27, 1967, Ser. No. 626,293 Int. Cl. H01h 9/30, 1/02 US. Cl.200-444 8 Claims ABSTRACT OF THE DISCLOSURE A vacuum circuit interruptercomprising an evacuated insulated housing having a pair of conductiveelectrodes which are constructed and arranged to be moved into and out.of engagement and circuit making and breaking regions on the electrodescomposed of a porous refractory such as W B for example, impregnatedwith a nonrefractory metal having high thermal and electricalconductivity wherein the refractory is readily wet by the nonrefractorywhen the latter is in its liquid state.

BACKGROUND OF THE INVENTION Vacuum circuit interrupters generallyinclude an evacuated insulating housing and circuit making and breakingcontacts constructed and arranged to be moved into and out ofengagement. In addition to the ability to interrupt fault currents andto withstand impulse crestvoltages across is open contacts, vacuumcircuit interrupters must be able to close against high momentarycurrents without producing objectionable welds between its contacts.Such welding is associated with the fluidizing of the metallic contactmaterial as a result of the heat pro duced during arcing. This liquidcontact material freezes after the contacts are brought into highpressure engagement, thereby forming a weld.

Oneprior art method for avoiding contact welds was to fabricate thecontacts out of a refractory metal such as tungsten. Such contacts werenot wholly satisfactory, however, because their are interrupting abilitywas relatively limited as a result of the low thermal conductivity ofthis material. Another prior art contact comprised a porous matrix of arefractory metal, such as tungsten, and a nonrefractory metal impregnanthaving high thermal and electrical conductivity, such as copper.Contacts of. this type were also satisfactory only for the interruptionof relatively low currents. At high current values the copper would tendto evaporate from the arcing surfaces so that, after a fewinterruptions, there remained a region of relatively pure tungsten. Thisregion would then heat to a relatively high temperature and emit aresustaining electrons by thermionic emission.

Ina further prior art attempt to solve the welding problem, the vacuuminterrupter contacts were fabricated of an alloy of copper-bismuth.Although this material is readily melted during arcing, it is weakphysically so that any resulting welds can easily be broken by theswitch opening apparatus. These contacts were not wholly satisfactory,however, because they are easily eroded during arcing and, therefore,their useful life is relatively limited. In addition, a large amount ofliquid contact material is lost by such contacts during arcing and thismaterial tendslto condense on the internal surfaces of the interrupter,to form sharp projections and other irregularities which tend to reduceits dielectric properties.

SUMMARY OF THE INVENTION Int general terms the invention comprises avacuum circuit interrupter in which at least one of its engageablecontact portions comprises a porous refractory including 3,514,559Patented May 26, 1970 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sideelevational view, partly in section, of a vacuum circuit interrupterillustrating how the contacts, according to the instant invention, maybe employed;

FIG. 2 is a view taken along lines 22 of FIG. 1;

FIG. 3 is a view taken along lines 3-3 of FIG. 2;

FIGS. 4 and 5 illustrate the method of determining the degree ofwetta'bility of a first metal in its solid state and a second metal inits molten state;

FIG. 6 illustrates the physical relationship between the components ofthe contacts according to the instant invention; and

FIG. 7 illustrates an alternate contact configuration in which thecontact material according to the invention may be employed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a vacuum-typecircuit interrupter 10 having an open-ended outer envelope 11 of anysuitable electrical insulating material, such as a ceramic, and upperand lower end caps 12 and 13, respectively, each of which is fixedlysecured at its outer periphery to one of the open ends of the envelope11 to form a vacuum chamber 14. A fixed electrode, or contact rod, 15 issealed to the upper end cap 12 and extends downwardly therefrom in asubstantially coaxial relation to the envelope 11. A second electrode orcontact rod 16, extends upwardly through an aperture 17 formed in thelower end cap 13 and is movable longitudinally into and out ofengagement with the fixed contact rod 15 by means which are not shownbut which are well known in the art. A bellows 18, or its equivalentstructure, is connected in a vacuum tight relation at each of itsopposite ends to the contact rod 16 and to the lower end cap 13 insurrounding relation to the aperture 17.

The contact rods 15 and 16 are axially aligned within the envelope 11and respectively carry arcing contacts 19 and 20 at their inner ends.The arcing contacts 19 and 20 may be generally disc-shaped and aredisposed in an opposed relation relative to each other. An arc shield 21may be affixed to the envelope 11 in surrounding relation relative tothe contacts 19 and 20 to prevent contact material, which may bevaporized during an are interruption, from depositing on the innersurface of the envelope 11 and thereby to provide a short-circuit patharound the electrodes 15 and 16. In addition a second, generallycup-shaped shield 25, is aflixed to the rod 16 for protecting thebellows 1-8.

The arcing contact 20 is preferably a mirror image of contact 19, andaccordingly, only contact 19 will be discussed in detail. Referring nowto FIGS. 2 and 3, the arcing contact 19 is shown to be generallydisc-shaped and to be affixed at its upper surface to the contact rod 15in any suitable manner, such as by brazing. The lower surface of thecontact 19 is provided with a generally circular recess 22 for receivinga relatively flat, circular disc-shaped member 23 which is secured inthe recess in a manner which will be discussed hereinbelow. Thethickness of the member 23 is greater than the depth of the recess 22 sothat it will be elevated relative to the remainder of the contact 19 andthereby will engage a corresponding member 23 of the lower contact 20.

A generally frustoconical support 26 is afiixed to the back of each ofthe contacts 19 and 20 to provide rigidity for said contacts.

The contact making and breaking regions 23, according to the invention,comprise a porous body, or matrix, composed of a refractory impregnatedwith a nonrefractory metal having high thermal and electricalconductivity wherein the nonrefractory tends to wet the refractory. Inaddition, it is preferable that the solubility of the refractory in theliquid state nonrefractory metal be substantially limited so that theporous matrix of the refractory matrix will be retained after repeatedarcings.

It is preferred that the refractory comprise a compound composed of ametal and a metaloid, and it is also preferable that the oxides of thernetalloid be solid at the normal operating temperatures of theinterrupter so that gases will not be liberated during arcing whichwould tend to destroy the vacuum. Two metalloids which form a refractorycompound with certain metals and which exhibit these characteristics areboron and silicon. These elements are also effective getters so thatwhen compounds of boron and a metal are employed in vacuum interrupters,it is possible to eliminate several of the expensive degassingoperations employed in the fabrication of certain prior art vacuuminterrupter contacts, such as those composed of copper andcopper-bismuth.

The nonrefractory metal having high thermal and electrical conductivityand which impregnates the refractory compound matrix may consist,according to the invention, of copper, silver, aluminum or any highthermal and electrical conductivity mixture thereof.

Certain compounds of boron and a metallic are unsatisfactory for use incircuit interrupter contacts because they have relatively low meltingpoints, such as NiB, which melts at approximately 1020" C. Preferablythe melting point of the compound should be in excess of 1750 C.

To illustrate certain refractory compounds of a metalloid and ametallic, reference is made to Table I wherein the melting points ofcertain illustrative compounds which may be used in circuit interruptersof the invention are given.

TABLE I.MELTING POINTS OF SOME ILLUS- TRATIVE REFRACTORY COMPOUNDSApproximate Compound: melting point C.)

crB 1850 MoSi 2050 Mo B 2100 MoB 2100 VB 2100 MoB 2100 WSi 2165 CeB 2190ThB 2195 LaB 2210 SrB 2235 BaB 2270 UB 236 5 W B 2770 WB 2900 TaB 2900NbB 2900 W13 2900 vB 2900 wB 2920 W B 2980 H113 3100 ZrB 3040 Certain ofthese compounds, which were tested and found to be highly satisfactoryfor high current applications, are ZrB W B, WB and W 8 Other compounds,such as CrB Mo B, MoB and MoB having a lower melting point, would besatisfactory at lower current applications.

Boron is known to form several other compounds with certain of thesemetals, such as ZrB, ZrB and ZrB For the sake of brevity, all of thesecompounds have not been listed.

The wetting action of a liquid metal on another material in its solidstate has been discussed in the literature. This phenomenon is reportedin terms of the angle that a droplet of the molten metal makes with aplanar surface of the solid state metal as illustrated in FIGS. 4 and 5.In FIG. 4 a droplet of copper at 1400 C. is shown to make an angle of 36with ZrB and in FIG. 5 a droplet of copper at 1400 C. is shown to makean angle of 154 with TiB Thus, because the molten droplet of copper wetsZrB it spreads out to form a relatively small angle with the planarsurface of the refractory. On the other hand, the molten copper dropletdoes not readily wet TiB so that it tends to assume a spherical shapewhereby a large angle is formed with the planar TiB surface. The anglesformed between molten cop-per droplets of some illustrative boroncompounds as reported in the literature are set forth in Table II.

TAB LE II Angle formed by drop of pure molten Cu on planar surface ofcom- Compound pound (deg) It can be seen from Table II that the abilityof molten copper to wet certain of the refractory compounds increasessubstantially as the temperature is increased from 1100 C., which isslightly in excess of the melting point of copper, to 1400-1500 C. Theability of molten copper to wet certain other compounds, however, suchas TiB is not appreciably increased by such an increase in temperature.This, while TiB is sufficiently refractory, having a melting point of2980 0., its use in the vacuum interrupter contacts according to theinvention is not preferred because it is not readily wet by moltencopper.

The ability of molten copper to wet certain refractory materials may beenhanced by the addition to the copper of certain wetting agents, suchas Ni. Such wetting agents should preferably be added only in amountswhich will not adversely effect the thermal and electrical conductivityof the copper. It may be noted that it is well known in the prior artthat the metals and metalloids which form the compounds illustrated inTable I are tightly bonded together so that very little of the boron orsilicon metalloids in solid form will be free to join with nickel toform the undesirable refractory compounds such as NiB. Also, it is knownthat the evaporated gaseous silicon or 'boron which combines in theatmosphere within the arc shield 21 with nickel or oxygen will mostlikely deposit in solid form on the arc shield or non-contactingsurfaces of contacts 19 and 20 because such surfaces are relativelylarge in comparison with the engaging portions of the contacts.

While the examples in the foregoing description have all involvedtheability of molten copper to wet various refractories, those skilled inthe art will appreciate that this will have application to silver andaluminum as well.

The arcing contact 23 is shown in FIG. 6 to comprise a porous body, ormatrix, 27 of the refractory compound impregnated with a nonrefractoryhigh thermal and electrical conductivity metal 28.

One illustrative process for forming the circuit making and breakingcontact 23 and involving the refractory compound W B and copper will nowbe discussed. A mixture of powdered W B and powdered copper is placed,in a die having the shape of the current making and breaking contacts23, and a pressure of approximately 50,000l00,000 p.s.i. is applied,although lower pressures are acceptable if the mold is suitably heated.This compresses the powdered mixture into a relatively solid mass. Thecompressed member is then placed into a vacuum furnace and heated at atemperature which is in excess of 1083" C., the melting point of copper,although a temperature in the order of 1700 C. is preferred. Thisthoroughly wets the refractory with copper and tends to cause therefractory particles to coalesce and grow together:to form a matrix. Theliquefied copper tends to fill the spaces between the W B matrix,although some copper is lost by evaporation.

Thecontact is then placed in a mold containing molten copper and heatedunder vacuum at a temperature in excess of the melting point of copper,but preferably at 1600-l700 C., for a period of sufficient length tothoroughly degas the copper. The temperature is then reduced toapproximately 1250 C. for a period of time sufficient to fill anyremaining voids with copper. Because the specific gravity of therefractory is substantially greater than that of the pure copper, thematrix member impregnated with copper does not tend to float in themolten copper. When the mold is cooled, a. composite contact member isthen provided which consists of the contact making and breaking member23 bonded to a backing of purse copper 19. The copper and thecopperimpregnated refractory may then be machined to the desired shapeshown in FIGS. 2 and 3. Small quantities of an impurity, such as a fewpercentage or less of silver or zirconium may be added to the copper toimprove its machinability;

This process insures that the spaces in the W B matrix are filled withcopper and also eliminates the necessity for thereafter brazing, orotherwise joining, the contact making and breaking electrode 23 to thecopper body 19..

While the foregoing process has been discussed with respect to W B it isintended that the same process would be employed with various otherhigher melting point compounds such as W B, WB, TaB NbB VB HfB and ZrBThe proportion of powdered refractory and copper 1n the original mixtureshould be such that there will be substantially continuous contactbetween the refractory particles. This result has been obtained withabout equal portions by volume of powdered copper and W 3 If the coppenportion exceeds about 60% by volume, then a continuousmatrix may not beobtained. If the proportion of copper falls below about 15% by volume,then the desired degree of thermal and electrical conductivity may notbe achieved. The preferred grain sizes are about 350 mesh or less forboth the copper and the refractory.

The desired results are also possible if mixtures of various powderedrefractories are employed, such as W B and ZrZB In addition, some minorproportion of a refractory metal such as tungsten may also be employed,although with less satisfactory results.

According ,to another embodiment of the invention illustrated in FIG. 7,the contact making and breaking material composed of the matrix of therefractory compound impregnated with a nonrefractory high thermal andelectrical conductivity metal covers the entire arcing face: of thecontact 30. Here the circuit making and breaking contact comprises athin lamination 31 of the arcing material according to the invention anda relatively thicker base 32l0f high thermal conductivity metal, such ascopper.

The contacts, according to the instant invention, provide most of theadvantages of refractory contacts and of high. thermal and electricalconductivity nonrefractory contacts while eliminating most of thedisadvantages thereof. Consider, for example, a contact composed of amatrix of W 3 impregnated with copper. Because the refractory W B isreadily wet by liquid copper, very little copper is lost during arcingeven though some of the copper may become liquid. As a result, thematrix remains impregnated with copper so that, even after repeatedarcings, there is a relatively high thermal and electrical conductivityat the contacting faces of the contacts and, in addition, heat generatedduring arcing will be rapidly conducted away from the arcing surfaces.Because of the relatively high melting point of the refractory compound,very little metal will be melted during arcing so that there will belittle tendency for contacts to weld upon re-engagement. In addition,because of the refractory nature of the matrix, there will be verylittle contact erosion even after repeated arcing so that a relativelylong contact life is realized. Also, because the solubility of W B inliquid copper is relatively low, the matrix will not dissolve to anygreat extent in any copper which may become liquid during arcing.Further, the current chopping level of contacts comprising a W B matriximpregnated with copper compares favorably with other vacuum interruptercontact materials such as an alloy of copper-bismuth which has a currentchopping level averaging between 4 and 5 amperes when the contactconfigurations shown herein are used.

While only a few embodiments and examples of the instant invention havebeen illustrated and described, these are only intended to heillustrative.

I claim:

1. A vacuum circuit interrupter comprising an evacuated envelope, a pairof contact members having engageable portions and being constructed andarranged for movement into and out of engagement, whereby a currentinterrupting arc is struck between said engageable portions, theimprovement wherein at least one of said engageable portions comprises aporous refractory material including a compound consisting of arefractory metal and a metalloid taken from the group consisting ofboron and silicon, and an impregnant consisting of a nonrefractory highthermal and electrical conductivity material, wherein the refractorymaterial is readily wet by the nonrefractory material when the latter isin its liquid state.

2. The interrupter set forth in claim 1 wherein the oxides of saidmetalloid are solid at the normal operating temperatures of theinterrupter.

3. The circuit interrupter set forth in claim 1 wherein said metal istaken from the group consisting of Ba, Ce, Cr, Hf, La, Mo, Nb, Sr, Ta,Th, W, U, V and Zr.

4. The circuit interrupter set forth in claim 1 wherein said metal istaken from the group consisting of Mo, Cr and W.

-5. The circuit interrupter set forth in claim 1 wherein said metalcomprises tungsten and said metalloid comprises boron.

6. A vacuum circuit interrupter comprising an evacuated envelope, a pairof contact members having engageable portions and being constructed andarranged for movement into and out of engagement, whereby a currentinterrupting arc is struck between said engageable portions, theimprovement wherein at least one of said engageable portions comprises aporous refractory matrix including a compound of boron and a metal takenfrom the group consisting of Mo, Cr, Zr, Hf and W impregnated with anonrefractory high thermal and electrical conductivity metal.

7. The circuit interrupter set forth in claim 6 wherein saidnonrefractory consists essentially of a metal taken from the groupconsisting of copper, silver and high thermal and electricalconductivity mixtures thereof.

8. A vacuum circuit interrupter comprising an evacuated envelope, a pairof contact members having engageable portions and being constructed andarranged for movement into and out of engagement, whereby a current 7interrupting arc is struck between said engageable por- OTHER REFERENCEStions, the improvement wherein at least one of said en- Hacklfs chemicalDictionary; by Julius Grant; third gageable Portia? matrix induding aedition; copyright, 1944, by the McGraw-Hill Book Comcompound ofzlrcomum and boron impregnated with a pany, Inc; p. 529 pertinentnonrefractory high thermal and electrical conductivity metal- ROBERT s.MACON, Primary Examiner References Cited UNITED STATES PATENTS

